The present invention relates to a transmissive body of high density polycrystalline alumina wherein the major surfaces have been polished with a molten fluxing agent to provide increased optical transmission. A tubular form of the flux polished material when used as the light transmissive envelope for an improved high intensity electric discharge lamp provides higher light-output than can be generally obtained with unpolished alumina material. The improved envelope material is particularly useful in high intensity sodium vapor lamp constructions of the type utilizing self heated electrodes, that is electrodes heated by the discharge and not by current from an external source.
High intensity sodium vapor lamps of the kind described in U.S. Pat. No. 3,248,590 - Schmidt, entitled "High Pressure Sodium Vapor Lamp," require the above type electrodes. These lamps further employ a slender tubular envelope of lighttransmissive, high density, polycrystalline alumina material which is resistant to sodium at high temperatures. A particularly suitable high purity alumina of this type is described along with methods for its preparation in a U.S. Pat. No. 3,026,210 - Coble wherein said material exhibits an in-line transmission of not less than 0.5% per millimeter thickness of the tube for radiant energy of all wavelengths in the wavelength region from about 0.3 micron to about 6.6 microns with an in-line transmission of not less than 10% at some wavelength within said wavelength range. Such polycrystalline alumina material generally contains a small but effective amount up to 0.5 weight percent magnesia to provide optical transparency wherein the magnesia content can be present primarily as an alumina-magnesia spinel. The filling in such high intensity sodium vapor lamps comprises sodium along with a rare gas such as xenon to facilitate starting and mercury for improved efficiency. Both ends of the alumina tube are sealed by refractory metal closure members, suitably niobium end caps bonded to the alumina tube with a glassy sealing material. Each end cap supports an electrode extending along the axis of the tube such as a tungsten rod having a double coil of tungsten wire wound around its inner end and with the electrodes further being coated with a suitable electron emissive material. A lamp of this construction is further described in a U.S. Pat. No. 3,708,710 - Smyser et al, along with a suitable method of lamp fabrication, so that it becomes unnecessary to repeat such details in the present specification.
It is also known to chemically polish opaque polycrystalline alumina objects in a fused borax bath so that dislocations and grain boundaries are not preferentially etched. The observed effect with this technique was a smooth and highly reflective surface being produced on the alumina material. Improved mechanical strength resulted from such treatment which is described in the technical article entitled "Chemical Polish and Strength of Alumina", by A. G. King, published in Vol.3, Materials Science Research (1966).